1. Field of the Invention
The present invention relates to a nonvolatile semiconductor memory device having a memory cell with a stacked gate structure.
2. Description of the Related Art
In the nonvolatile semiconductor memory device such as a NAND type flash memory, each memory cell (cell transistor) has a stacked gate structure.
The stacked gate structure means the structure in which a floating gate electrode and a control gate electrode are stacked on a channel in a source/drain diffusion layer. Between the channel and the floating gate electrode, there exists a first insulating film called “tunnel insulating film”. Between the floating gate electrode and the control gate electrode, there exists a second insulating film called “inter-polysilicon dielectric” or “inter-electrode insulating film”.
In the conventional technique, a stack insulating film of SiO2/Si3N4/SiO2 called ONO film is mainly used as the second insulating film (for example, refer to JP-A 2003-68897 (KOKAI), 2003-197785 (KOKAI), and 2004-281662 (KOKAI)).
A memory cell in the future, in pursuit of miniaturization for larger memory capacity, needs to further decrease the equivalent oxide thickness of the second insulating film. For this purpose, it is examined that Si3N4 in the central portion of the ONO film is replaced by the material of higher dielectric constant (high-k material). Namely, SiO2/high-k/SiO2 is proposed to form the second insulating film.
The Si3N4 or high dielectric constant (high-k) insulating film, however, in the central portion of the ONO film and/or the SiO2/high-k/SiO2 film originally includes a lot of defects. Since the defects of the Si3N4 film are related with nitrogen concentration, they seem to be caused by nitrogen.
On the other hand, it is known that the defects of the high-k insulating film are mainly caused by those centers having energy levels in a conduction band of the high-k insulating film (for example, refer to H. Takeuchi, D. Ha, and T.-J. King, “Observation of bulk HfO2 defects by spectroscopic ellipsometry,” J. Vac. Sci. Technol. A22, 1337 (2004)).
This fact originates from the d-orbital included in a metal element of a high-k insulating film (for example, refer to G. Lucovsky and J. L. Whitten, “Chemical bonding and electronic structure of high-k transition metal dielectrics: applications to interfacial band offset energies and electronically active defects,” pp. 325-371 (Chapter 4.2), in High-k Gate Dielectrics, Edited by M. Houssa, IOP Publishing Ltd (2004)).
The ONO film and the SiO2/high-k/SiO2 film can suppress the detrapping of the trapped charges, by the presence of the SiO2 films at both ends of the central defective insulating film hence to secure electrical stability in the stacked gate structure.
As an adverse effect, however, because of the SiO2 films existing on the both ends, the second insulating film cannot help but increase the equivalent oxide thickness, which interferes with its use for a memory cell of the future generations requiring scaling down of the thickness. On the contrary, when the SiO2 films existing on the both ends are removed, the second insulating film can be reduced in the equivalent oxide thickness, but the detrapping of the trapped charges remarkably occurs, resulting in a change in the threshold voltage of the stacked gate structure, hence to deteriorate the data retention characteristic of the memory cell.